In automation technology, field devices, in particular so-called two-wire field devices, are often utilized in areas in which the power consumption of a field device is restricted to a predetermined value. An example of this is a potentially explosive area in an industrial manufacturing or process plant in which automation devices are used. In areas such as these, limit values are prescribed for the maximum power consumption of a field device. The relevant provisions in this regard are determined by various organizations and associations at a national and international level. Two-wire field devices are particularly suited for use in such areas due to their limited power consumption.
Various types of protection, such as protection by means of pressure-encapsulated enclosures or encapsulation in sand, potting compound, or oil, whereby explosion protection is ensured by means of constructive measures which aim to prevent conductive connections from protruding into the potentially explosive medium, are known for use in the explosion-protected area.
Intrinsically safe circuits are here generally subject to stringent restrictions with regard to operating voltage and the electrical power available to operate the circuit. The measures for limiting the ignition energy in the event of a fault also limit the performance values available for normal operation and, as a rule, prevent the operation of a circuit which requires high short-duration peak currents to function.
Automation field devices are widely used in industrial plants. For example, field devices, which are used to record and/or influence process variables, are widely used in process technology, as well as in automation technology. Measuring devices, such as level measuring devices, flow meters, pressure and temperature measuring devices, pH meters, conductivity meters etc., are used for recording process variables and record the corresponding process variables for level, flow, pressure, temperature, pH and conductivity. Actuators, such as valves or pumps, are used for influencing the process variables by which, for example, the flow rate of a liquid in a pipe or the fill level of a medium in a container is changed. In principle, a field device is any device which is used close to a process and which provides or processes process-relevant information. A number of such field devices is offered and sold by the Endress+Hauser group of companies. All types of measuring devices and actuators should therefore be subsumed under the term field device as used in conjunction with the invention. Furthermore, the term field device also includes, for example, a gateway, a wireless adapter or other bus subscribers which are or can be integrated into a bus system.
In this connection, the necessity of commissioning and the configuring of field devices should be noted. For example, when a field device is calibrated, a communication path needs to be established from an operator panel to the field device itself or between field devices. However, this task can require considerable effort, particularly in potentially explosive plants. It is important to minimize the non-productive time of a plant or a section of a plant caused by a configuration or maintenance process. For this reason, wireless solutions, i.e. the use of wireless technologies for communication between field devices or between an operator panel and a field device, are to be considered preferable to a cable connection to an industrial interface of a field device.
A wireless field device that can be used in potentially explosive areas is known from German Patent, DE 600 18 072 T2. In a preferred embodiment, which is disclosed in this publication, the reduced availability of power or energy is taken into consideration. An energy buffer, which prestores energy during the period in which the transmitter unit is not sending out any radio signals, is provided in the field device to provide the energy required for sending out wireless signals. It is further provided for the wireless to be stopped when the energy supply is no longer sufficient. This allows error-free data transmission which would not otherwise be possible were a radio transmission to be terminated. However, the field device must be completely redesigned to enable wireless communication.
In accordance with prior art, many different wireless dongles are known which can create or expand the wireless capability of a field device. For example, USB wireless dongles exist which can be connected to a USB interface of a computer and used for wireless communication with other field devices and/or control panels. However, such wireless dongles are not suitable in any way for potentially explosive areas in industrial plants since the performance characteristics and Ex-safety properties do not meet the requirements described above.